Changes of trophic conditions in the northern Adriatic Sea

The northern Adriatic Sea (NAd) is the shallowest, land locked, northernmost part of the Mediterranean Sea. Water column stratification caused by buoyancy input and heat fluxes usually takes place in spring and summer, but intense mixing and water column homogenization regularly occurs during colder seasons, because of the exposure to strong and cold easterly winds and of winter dense water formation. Nutrient inputs from Italian rivers are more concentrated along the western and northern coasts, ,although they can sustain plankton activity over a much more extended area. Changes in the trophic conditions in the NAd ecosystem have been observed, based on analyses of data time series on riverine discharge rates, oceanographic features, and biological components, collected since the 1970s. A gradual increase of eutrophication pressure occurred during the 1970s up to the mid 1980s, followed by a tendency toward oligotrophication, particularly marked in the 2000s. This tendency was ascribed to the combination of a reduction of the anthropogenic impact (mainly a substantial decrease of the phosphorus load) and climatic variations, resulting in a decline of atmospheric precipitations and, consequently, of the runoff in the NAd watersheds. Significant decreases of the phytoplankton abundances were observed after the mid 1980s, concurrently with changes in the species composition of the communities, with an evident shift to smaller cell or organism sizes.